 |
 |
Brandall Y. Suyenobu wrote: > Hello SPM users > > Is there a consensus as to the optimal way of analyzing multiple fMRI > runs of a single subject as separate sessions? Probably not ! But I'll offer my opinion ! > In a recent post the > separate session analysis was pointed out as the "normal procedure" > http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind0302&L=spm&P=R33056&I=-3 > <http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind0302&L=spm&P=R33056&I=-3>, > and previous posts (e.g., > http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind0207&L=spm&P=R13130&I=-3) > have pointed out that, although valid, across-session contrasts are > usually less statistically efficient; I think Rik's (Henson) point here is that across-session contrasts are less statistically efficient because the effects are further away in time - and therefore will not be so well matched (matched comparisons being more sensitive than unmatched ones). But this point is a critique of the design - not on how the data are analysed ! ie. if you model the sessions separately or together it won't make the effects any closer together in time. > however, in the case of the > multiple-run-single-subject analysis, there seems to be little > alternative with some experimental designs. In a current study we are > obtaining 6 10-min scan series (runs) per subject per session, and > within each run presenting the subject with 3 active conditions and 2 > rest conditions in a total of 16 presentations. When analyzing this as > separate sessions, is it best to employ second level analyses with the > relevant contrasts when, e.g., summarizing condition effects over all 6 > runs? > If you have a multiple subject - multiple session (per subject) study then it is possible to do RFX analysis both over sessions and then over subjects. If you did do this then you would be applauded for your rigor but I think most people would do FFX over sessions and then RFX over subjects - this is because the between-subject variance is likely to be larger than the between-session variance (although this is an empirical issue related to a number of factors including eg. within-session sample size). If, however, you are doing a single subject (or a small number of subjects eg. 3) case-study then RFX over sessions is a reasonable option. > And, alternatively, is there an acceptable (i.e., least problematic) > method of analyzing multiple runs of a single subject as a single > session? In our study, the TR of 3 sec yields 200 scans per run that I > have analyzed as a single session (fixed response/box-car analysis), > 1200 scans, with, I think, reasonable results. > As I am not modeling the > rest conditions, the sum of the time between analyzed segments exceeds > the inter-session interval does this have a bearing on the > appropriateness of a single-session analysis? Any comments will be most > welcomed. > > Best regards, > > Brandall Y. Suyenobu, Ph.D. > Staff Research Associate > > Brain Imaging Group > CNS: Center for Neurovisceral Sciences & Women's Health > CURE: Digestive Diseases Research Center > UCLA Division of Digestive Diseases > Greater Los Angeles Veterans' Administration Health Care, > Bldg. 115, rm 223 > 11301 Wilshire Boulevard > Los Angeles, California 90073 > Tel: (310) 478-3711 ext. 40580 > Fax: (310) 794-2864 > http://ibs.med.ucla.edu <http://ibs.med.ucla.edu/> > http://mindbody.med.ucla.edu <http://mindbody.med.ucla.edu/> > -- William D. Penny Wellcome Department of Imaging Neuroscience University College London 12 Queen Square London WC1N 3BG Tel: 020 7833 7478 FAX: 020 7813 1420 Email: [log in to unmask] URL: http://www.fil.ion.ucl.ac.uk/~wpenny/